1. Field of the Invention
The invention relates to an apparatus and method for filtering inclusions from molten metal alloy. More particularly, the invention relates to filtering inclusions from a molten metal alloy prior to extrusion from a planar flow casting nozzle to form solid glassy metal alloy filament.
2. Description of the Prior Art
When casting metal alloys such as aluminum and iron alloys, inclusions comprised of oxides and nonmetallic particulates can form in the melt and be held in suspension. During casting operations, the inclusions can accumulate and adhere to the walls of the casting nozzle near the outlet orifice. The accumulation can then disrupt the stream of molten metal, disrupt the geometry of the cast material and clog the casting nozzle. If the inclusions pass through the nozzle, they can degrade the properties of the cast metal. An example of this phenomena is discussed in "Mechanism of Alumina Buildup in Tundish Nozzle During Continuous Casting of Aluminum-Killed Steels" by S. N. Singh, published in Metallurgical Transactions, October, 1974 issue.
Filter beds of tubular alumina, alumina balls and ceramic foam sponge have been used to filter molten aluminum prior to casting, as discussed in the paper, "A Laboratory Investigation of Aluminum Filtration Through Deep Bed and Ceramic Open-Pore Filters" by R. Mutharasan, D. Apelian, and C. Romanowski, published in Journal of Metals, December, 1981 issue. Methods and apparatus, including casting nozzles, for casting glassy metal filament are disclosed in U.S. Pat. No. 4,142,571 for "Continuous Casting Method for Metallic Strips" issued Mar. 6, 1978 to Narasimhan.
Inclusions have been particularly troublesome when casting filaments of glassy metal alloys because the glassy metal alloys are cast at extremely high speeds, typically up to 2,200 m/min; the exit orifice of the casting extrusion nozzle is very narrow, usually about 15 to 20 mils (0.038-0.051 cm) wide; and because the cast metals are very thin, 25-100 microns in thickness. Consequently, any inclusions that collect in the casting nozzle outlet orifice will seriously degrade the cast metal. There will be unacceptable variations in the filament cross-section, and the cast filament may split and become discontinuous across its width dimension. As a result, the casting operation often becomes disrupted within a matter of minutes after starting.
Conventional filters, such as ceramic sponge filters, have randomly sized and randomly located filtering pores. Conventional filter beds are typically constructed with different size filter particles located randomly throughout the bed or constructed with progressively smaller filter particles located downstream from relatively larger filter particles. Such filters and filter beds have proven to be unsatisfactory because fine particulates released by the filter material itself migrate out of the filter, deposit onto the nozzle walls and clog the nozzle orifice. As a result, casting operations using conventional filters have had higher costs and lower yields than are considered to be acceptable.